1. Field of the Invention
The present invention relates to a fuel cell casing that is capable of accommodating a membrane electrode assembly, is made of ceramics, and is small and highly reliable, and to a fuel cell using the same.
2. Description of the Related Art
In recent years, development of compact fuel cells that are operable at a lower temperature than ever before has been briskly under way. Fuel cells are classified according to their electrolytes in use. For example, there have been known Polymer Electrolyte Fuel Cell (hereinafter abbreviated to “PEFC”); Phosphoric-Acid Fuel Cell; and Solid-Oxide Fuel Cell.
Among them, in particular, the PEFC is operable at a temperature as low as 80 to 100° C., and also possesses many excellent characteristics as follows:    (1) its power density is high, and miniaturization and weight reduction are allowed;    (2) since an electrolyte membrane is not corrosive, its operation temperature is low, and therefore the constitution material of the cell is constrained little from the aspect of corrosion-resistance, cost reduction is easy; and    (3) actuation at ordinary temperatures is allowed, and therefore, actuation time is short. By taking advantage of such characteristics, it has been considered to not only apply the PEFC to driving power sources for a vehicle, household cogeneration systems and the like but also use as power sources for mobile electronic apparatuses such as mobile phones, PDAs (personal digital assistants), notebook-type personal computers, digital cameras, videos, and the like whose outputs are a few watts to several tens of watts.
Roughly, the PEFC is composed of a fuel electrode (anode), an air electrode (cathode), and a film-shaped membrane electrode assembly interposed between the fuel electrode and the air electrode. The fuel electrode is formed of a carbon electrode having catalyst fine particles of platinum, platinum-ruthenium, or the like attached thereto. The air electrode is formed of a carbon electrode having catalyst fine particles of platinum or the like attached thereto. Here, the fuel electrode is supplied with hydrogen gas (H2) extracted through a reforming section, whereas the air electrode is supplied with oxygen gas (O2) present in the air. Through an electrochemical reaction, electric energy of certain level is generated (electric power production), and thereby electric energy acting as driving power (voltage/current) for a load is produced.
Specifically, when hydrogen gas (H2) is supplied to the fuel electrode, as shown in the following chemical equation (1), with the action of the catalyst, an electron (e−)-separated hydrogen ion (-proton; H+) is generated, and the proton passes through the membrane electrode assembly toward the air electrode. Simultaneously, the electron (e−) is ejected by the carbon electrode constituting the fuel electrode and is then supplied to a load.3H2→6H++6e−  (1)
On the other hand, when air is supplied to the air electrode, as shown in the following chemical equation (2), with the action of the catalyst, the electron (e−) having reached the load and the hydrogen ion (H+) having passed through the membrane electrode assembly, and oxygen gas (O2) present in the air react with one another to form water (H2O).6H++3/2O2+6e−3H2O  (2)
Such a series of electrochemical reactions (refer to equations (1) and (2)) commonly take place at a relatively low temperature of approximately 80 to 100° C. Basically, a by-product material other than electric power is water (H2O) alone.
As an ionically conductive membrane (Polymeric solid electrolytes) constituting a membrane electrode assembly, there have hitherto been known a cation-exchange membrane composed of the polystyrene-base having a sulfonic acid group, a mixture membrane of fluorocarbon sulfonic acid and polyvinylidene fluoride, a membrane obtained by grafting trifluoroethylene to a fluorocarbon matrix, and the like. In recent years, for example, a perfluorocarbon sulfonic acid membrane has been in use (available from DuPont Co. under a trade name “Nafion”).
FIG. 4 is a sectional view showing the structure of a fuel cell (PEFC) of conventional design. In the figure, reference numeral 21 denotes the PEFC, reference numeral 23 denotes a membrane electrode assembly, and reference numerals 24 and 25 denote a pair of porous electrodes that are arranged on the membrane electrode assembly 23 so as to sandwich therebetween the membrane electrode assembly and that have functions as a gas diffusion layer and a catalyst layer, that is, a fuel electrode and an air electrode, reference numeral 26 denotes a gas separator; reference numeral 28 denotes a fuel duct; reference numeral 29 denotes an air duct; reference numeral 30 denotes current collecting plates; reference numeral 31 denotes clamping plates; and reference numeral 32 denotes screws.
The gas separator 26 is composed of a stack portion; a gas inlet/outlet frame; a separator portion; and electrodes. The stack portion and the gas inlet/outlet frame constitute the outer shape of the gas separator 26. The separator portion serves to separate the fuel duct 28 and the air duct 29. The electrodes are disposed so as to pierce the separator portion and placed so as to correspond to the fuel electrode 24 and the air electrode 25 of the membrane electrode assembly 23. A multiplicity of membrane electrode assemblies 23 are stacked on top of one another via the gas separators 26, in such a way that the fuel and air electrodes 24 and 25 of the membrane electrode assembly 23 are connected in series and/or in parallel with one another through electrical connection. Electric power is obtained by means of the current collecting plates 30. The gas separator 26 is clamped down by the screws 32 at an adequate surface pressure with use of the clamping plates 31. Thereupon, a fuel cell stack, i.e., a minimum unit of a cell, is constructed. By accommodating the fuel cell stack in a casing, a general PEFC main body is realized.
Fuel gas that contains water vapor (gas that is rich in hydrogen) is supplied from a reforming device to the fuel electrode 24 through the fuel duct 28 formed in the gas separator 26 and the air is supplied as oxidant gas from the air to the air electrode 5 through the air duct 29, and electric power is generated by a chemical reaction in the membrane electrode assembly 23.
There is Japanese Unexamined Patent Publication JP-A 2001-266910 and Japanese Unexamined Patent Publication JP-A 2001-507501 as the related art.
However, this fuel cell 21 that has been proposed and developed up to now as a high-voltage and high-capacity cell is a heavy and large cell which has a stack structure and whose constitution elements have large areas, and use of a fuel cell as a small cell has been hardly considered so far.
Specifically, the conventional gas separator 26 disposed in the fuel cell 21 poses a problem that since the side surfaces of the membrane electrode assemblies 23 are exposed outside in a stacked body made by stacking the membrane electrode assemblies 23 by the use of the gas separator, the construction is easily damaged because of a fall at the time of carrying, and it is hard to guarantee mechanical reliability of the whole fuel cell 21.
Furthermore, in order to install the fuel cell 21 in mobile electronic apparatus, a fuel cell casing that is excellent compactness, convenience and safety unlike a conventional large fuel cell casing is necessary. In other words, it is necessary, in order to apply as a portable power source such as a general-purpose chemical cell, to miniaturize and low-profile a fuel cell casing for the purpose of shortening time for increasing temperature up to operation temperature and making a thermal capacity small. However, the gas separator 26 that dominates a large proportion of a thermal capacity in the conventional fuel cell 21, specifically, the gas separator 26 where the ducts are formed on the surface of a carbon plate by cutting processing becomes fragile when becoming thin-walled, and therefore, it needs thickness of a few millimeters. Therefore, there is also a problem that it is hard to miniaturize and low-profile.
There is another problem associated with the output voltage of the fuel cell 21. In a case where the gas separator 26 is made larger in thickness, resistance is increased, and thus voltage losses with respect to output current is increased. As a result, the output voltage is decreased. Further, combinations of a plurality of membrane electrode assemblies 23, the opposed fuel electrodes 24 and air electrodes 25, and the gas separators 26 need to be arbitrarily connected together in series or in parallel with efficiency so as to adjust the output voltage and the output current as a whole. In the conventional fuel cell 21, in order to obtain electric power from the fuel electrode and the air electrode, sandwiching therebetween the membrane electrode assembly 23, such a method is adopted that the current collecting plates 30 are connected with an external electric circuit, and the membrane electrode assemblies are stacked through a plurality of gas separators 26 as conductive materials, and are connected in series by means of the clamping plates 31. In this case, quite inconveniently, the current collecting plate 30, the clamping plate 31, and the screw 32 need to be electrically insulated from one another. Thus, the conventional fuel cell, when made compact, presents the problems of an increase in the number of constituent components; the difficulty in slenderization; and poor flexibility in establishing electrical connection among the individual cells.